A cellular wireless network typically includes a number of base stations that are configured to provide wireless coverage areas in which user equipment devices (UEs) such as cell phones, tablet computers, machine-type-communication devices, tracking devices, embedded wireless modules, and/or other wirelessly equipped communication devices (whether or not user operated) can operate. Each base station could be coupled with a core network that provides connectivity with various application servers and/or transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the cellular network could engage in air interface communication with a base station and could thereby communicate via the base station with various application servers and other entities.
Such a network could operate in accordance with a particular radio access technology (RAT), with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link.
Over the years, the industry has embraced various generations of RATs, in a continuous effort to increase available data rate and quality of service for end users. These generations have ranged from “1G,” which used simple analog frequency modulation to facilitate basic voice-call service, to “4G”—such as Long Term Evolution (LTE), which now facilitates mobile broadband service using technologies such as orthogonal frequency division multiplexing (OFDM) and multiple input multiple output (MIMO). And most recently, the industry is now exploring developments in “5G” and particularly “5G NR” (5G New Radio), which may use a scalable OFDM air interface, advanced channel coding, massive MIMO, beamforming, and/or other features, to support higher data rates and countless applications, such as mission-critical services, enhanced mobile broadband, and massive Internet of Things (IoT).
In accordance with the RAT, each coverage area could operate on a carrier (carrier frequency), which could be frequency division duplex (FDD) or time division duplex (TDD). An FDD carrier would use separate frequency channels for downlink and uplink communication, whereas a TDD carrier would use a single frequency channel multiplexed over time between downlink and uplink use. In either case, the downlink and uplink would each typically span a particular frequency bandwidth, defining within the frequency bandwidth various air interface resources for carrying communications between the base station and served UEs. Further, the air interface resources could be grouped or otherwise structured to define various logical channels for carrying control signaling and bearer and/or other traffic between the base station and UEs.
In a system that supports TDD operation, each coverage area that operates on a TDD carrier could be configured with a frame structure that has a TDD configuration (frame configuration) defining a sequence of equal-duration subframes and establishing which subframes are for downlink use and which subframes are for uplink use. In some cases, special subframes for transition between downlink and uplink operation could be provided as well.
By way of example, a coverage area could operate on a TDD carrier, and this carrier could have a TDD configuration that establishes for each subframe per frame whether the subframe is a downlink subframe (D), an uplink subframe (U), or a special subframe (S). For instance, the TDD configuration could be {D, S, U, U, D, D, S, U, U, D} or {D, S, U, D, D, D, S, U, D, D}, among other possibilities. Further, the base station that provides such coverage could broadcast a system information message that specifies details of the TDD configuration, to enable served UEs to operate accordingly.